Currently, cranksets comprise at least one chainring with ring gear which is secured in rotation with a shaft rotating about a fixed pedal axis and which drives a chain. The latter in turn drives a sprocket, secured to a rotation shaft of the rear wheel. This sprocket is not necessarily unique, it may consist of derailleur gears equipped with several sprockets of different ratios to adapt to the terrain configurations. The chainring is secured to two cranks symmetrically located on either side of the bicycle frame and offset from each other by 180°. The free ends of the cranks are fitted with a screw thread for attaching a pedal free to rotate about a connecting axis. As a result, by exerting pushing force on the pedals, the cyclist rotates the chainring which in turn drives the rear wheel by means of the chain. This type of crankset has disadvantages, the main one is due to the symmetrical configuration of the cranks which have their longitudinal axes aligned. In particular, when one of the pedals is in the low position (bottom dead center: BDC), its opposite is in the up position (top dead center: TDC). If the cyclist pushes down on the pedal in this position, this thrust is clearly ineffective since there is no force component to rotate the cranks.
In the state of the art, there are different solutions to facilitate the passage of the top dead center (TDC). The most relevant systems use independent cranks and a chainring decentered by a cam. Connecting rods provide the connection between the cranks and the chainring. This is described, for example, in patent FR 2 584 671, in patent application FR 2 742 408 A1, in application US 2004/045401 A1 or in U.S. Pat. No. 6,367,352 B1.
In these mechanisms, there are disadvantages due to geometry. The articulation axis on the crank which controls the rotation of the chainring is located between the pedal axis and the crank axis. This configuration requires, for the removal of the dead center, a forward decentering of the chainring in a position that is not stable. The traction of the chain on the crankset tends to bring the decentering cam backwards. It is therefore necessary to provide a system for maintaining the cam in its forward decentering position. Additionally, the chain is extended by this forward decentering which causes an increase in the weight of the cycle.